If you don't know what the metal is there is no way to know if it's dangerous to swallow. I suggest that stainless steel is not toxic and if it's stainless in the bridge and pieces have broken off I suggest it's because it was worn out.
My grandson has a couple of metal crowns. I have a couple of porcelain crowns toward the front of my mouth but there is metal underneath. The back crowns have to be entirely metal to withstand the force of chewing. The only alternative to stainless steel, which I think is what they use, is gold. Gold crowns are very expensive. Because of the expense, they do not use gold on baby teeth. Metal for crowns have been used for years; probably a century or so and I don't think anyone has found that they are toxic.
Amalgam fillings are now thought to be toxic but those are very much different than crowns.
If you don't take him to the dentist at least every 6 months or so, I suggest that the reason the bridge fell out is because not only did he outgrow it but it also wore out. It is extremely important that a new bridge be put in so that his permanent teeth will come in straight.
He did need crowns on at least the front two molars to hold the bridge which are his fake front teeth. If his front teeth were pulled because they had cavities or were rotten then the back teeth would also have had cavities. Ask the dentist why he crowned the rest?
Please relax while you research to get more info. Good to take him to see the pediatrician but I'd suggest that his dentist would be a better choice. The dentist knows what the metal is made of and whether or not it's toxic. Once you know what the metal is you can look it up on the Internet and find out for yourself if it's likely to hurt your son. I
If you're concerned about his swallowing pieces of something, we all, at one time or another have swallowed metal pieces. Babies and toddlers put everything in their mouths. The metal goes thru the digestive track and out in the feces. Only heavy metals such as lead can be toxic when breathed into the lungs where they stay and contaminate the rest of the body thru the blood system. When lead is swallowed, the doctor does sometimes take an x-ray to be sure that it does pass thru. But small pieces of stainless will not harm your son. They are long gone, if he swallowed them at all.
In the meantime, do not panic. I've yet to hear of anyone, child or adult, to be harmed by the metal in a bridge. My mother had one all of her adult life and had to replace it several times. The bridge caused her no health issues.
Also, you do have to be careful with a bridge. If your son was using those face teeth to bite down on things, say an apple or a toy car, the bridge will break and that may be what has caused the small pieces. Please call the dentist and tell him your concerns. I suggest that the pediatrician will want to know what type of metal it is, also. I doubt that they can run a test unless they know what it is
Do you brush your son's teeth or see that he does twice/day? The black spots could be corrosion from the food acids left in his mouth. Regular dental care to include twice daily brushing is essential for healthy teeth and thus a healthy body. By caring for his teeth daily, he will most likely have healthy permanent teeth within a few years and the metal will be gone.
I reread your post and saw that you have talked with the dentist who told you that it's normal. Why do you not believe him? Why do you think that the metal is bad for your son? His teeth are not healthy. Do you know why he had to have his front teeth pulled? Do you have a solution for fixing his teeth if he has no metal in his mouth? Are you willing for him to grow up without teeth because he doesn't have metal in his mouth. Compare the results of having metal and no metal. Which is the "worst of two evils." I call them evils because you think metal is a bad way to go and the alternative is growing up with poor teeth or no teeth at all. His permanent ones can rot while under the gum and all of his teeth but especially his front teeth will grow in crooked without baby teeth, crown, or a bridge to hold the space for the permanent teeth.
After your So What Happened, T.: Wow! this response is so different then the way the question was worded. You obviously have done some research and know more than I do about metals and dentists. In your question you said you didn't know what the metal was. If you'd named the "stuff" my answer might have have been different. After your What Happened, I wonder why you aren't more knowledgeable about the causes for the caps on his molars. You sound, now, like the sort of person who does ask good questions and makes informed decisions.
Did you ask why the caps and wait until you got an answer that you understood. I wonder how you can be so sure that there weren't any cavities that would cause the need to cap his teeth. A tiny bit of decay can cover a large amount of decay beneath the enamel. Also, some of us are born with really soft enamel. If your son has this condition they should have talked with you and gained your permission before doing this.
Oh, well. I'm glad that you're going to a metal free dentist. I didn't know there were such dentists either.
Obviously you don't trust the one who did the work. Perhaps that's why you're so scared now. Have you tried asking, with an open mind, for the reasons behind the answers? If all that you describe is true without adequate explanations that would cause other dentists to agree with the decision, then you may have grounds for a law suit. Apparently, what is happening to you and your son is much more complex then anything with which I've had experience. I've found that conditions such as you describe are usually caused by bottle mouth and were documented by me and a dentist. My grandson has capped teeth and 3 fillings because he was given too much candy and has multiple caretakers due to a divorce and his teeth were not adequately brushed nor was he taken often enough to the dentist. I'm glad your son is an exception. However, I still go back to the weak enamel suggestion.
I would rely on the metal free dentist's recommendation. I doubt that any of us have sufficient personal experience or knowledge about metal and it's effect on your son's health to know how to advise you. I suggest that the fact that he will be loosing those teeth in the next few years might make a difference in the level of concern over toxicity. I suggest you also have to factor in the possible long term effects of general anesthesia and in disturbing his teeth and gums.
I wish you the best as you work this out. Your post has caused me to do some research for my own dental work. Thanks for posting it.